Field of the Invention
The present invention relates to an alignment device, a splicing device, an aligning method, and a splicing method for optical fibers having two or more cores such as multi-core fibers.
Description of Related Art
In recent years, internet traffic has continued to increase, and thus, the transmission capacity for an optical fiber configuring a communication network is required to be increased. As a key technology in this area, a technology of a multi-core fiber in which one optical fiber has a plurality of cores configuring transmission paths has attracted attention.
Since multi-core fibers have characteristics of having cores at various positions in the cross-section of the fiber, research and development relating to the input and output technology of the multi-core fiber is being actively carried out.
As a method for splicing optical fibers, a fusion splice using a fusion splicer can be included.
A general aligning method using a fusion splicer in the related art is as follows.
Firstly, a reference light is launched into a side surface of a bare fiber of which the coating is removed to observe an internal structure from the side surface, then, a boundary between the core and cladding can be recognized from a refractive index difference between the core and the cladding. Therefore, the boundaries between the core and cladding of two bare fibers facing each other are matched by a two-axis operation in the x-axis and the y-axis, and then, the two bare fibers can be aligned.
On the other hand, in splicing multi-core fibers, each of the plurality of cores is required to be aligned. That is, it is necessary to match the core arrangements in the cross-sectional surfaces of fibers facing each other. However, in the side surface observing method in the related art, there are many problems such as a problem in which it is difficult to recognize the boundaries between the core and cladding for all the cores in the multi-core fiber because the plurality of cores overlap in the front of and behind in the observing direction.
Therefore, as the splicing method for multi-core fibers using the fusion splicer in the related art, there is a method in which, after roughly aligning the cores based on the outer diameter of the fibers, while actually causing measurement light to pass through any of the cores and measuring the transmittance, the cores are aligned until the transmittance becomes a maximum (the splice loss becomes a minimum) (for example, refer to Patent Document 1 (Japanese Unexamined Patent Application, First Publication No. 2013-54116)).
As another example of the aligning method, a method of giving a specific marker to multi-core fibers is known (for example, refer to Patent Document 2 (Japanese Unexamined Patent Application, First Publication No. 2011-158768) and Patent Document 3 (Japanese Unexamined Patent Application, First Publication No. 2013-50695)).
In addition, even though the method described below is not related to multi-core fibers but related to optical fibers having a plurality of holes extending parallel in the cores such as a photonic crystal fiber, a method is disclosed in Patent Document 4 (Japanese Unexamined Patent Application, First Publication No. 2004-53625), in which a position of end surfaces of optical fibers are observed and the positions of the optical fibers are adjusted such that cores of both the optical fibers are on the same axis and the holes are arranged so as to overlap each other.
The splicing method in the related art is a method in which, being limited to multi-core fibers having completely the same design, two specific points are aligned, and then, all the cores can be aligned. This concept is based on a mathematical theory that, when two points in a diagram existing on a plane space are determined, ideally, the position of the diagram is uniquely determined. Therefore, when the technology in the related art is used, ideally, the cores of the two optical fibers to be spliced are completely matched, and the splice loss becomes a minimum value. However, this technology in the related art is based on the assumption that the core positions in the end surfaces of the two optical fibers facing each other completely match.
In the actual splicing of multi-core fibers, the core positions in the end surfaces of the fibers deviate from the designed positions. A deviation with respect to the designed positions cannot be avoided in manufacturing. However, in a real situation, the deviations are not inherently assumed and are randomly distributed. Therefore, in a case where the deviations in the core positions are considered, according to the related art, it is possible to minimize a value of splice loss for specific arbitrary cores selected at the time of aligning. However, with regard to the other cores, there is a possibility that the positional relationships between the specific selected cores and the other cores in the fibers are not the same due to the influence of the random deviations. Therefore, even when the alignment is performed such that the value of splice loss for the specific selected cores is minimized, since the splice loss for other cores may become larger than the minimum value, the average splice loss for all the spliced cores does not always become the minimum value.
Of course, by slightly moving the optical fibers facing each other while the reference light is incident on all the cores, it may be possible to search for the position where the total sum of the splice losses of all the cores becomes a minimum value, but this is not so realistic and is very difficult.
In the methods disclosed in Patent Document 2 and Patent Document 3, the marker needs to be given to the multi-core fibers in advance, and thus, these may not be highly versatile methods.
In the method disclosed in Patent Document 4, if the positions of each core do not deviate from each other, it is possible to search for the positions where the cores overlap each other. However, in a case where the positions of each core deviate from each other and an axial deviation cannot be avoided from being generated on a part of the cores, it is not possible to obtain the position where all the cores overlap each other.
Briefly describing these problems in the related art, all of these technologies are very effective methods in a case where the core positions of the multi-core fibers are completely the same. However, in the multi-core fibers as an actual information transmission path, it is considered that there cannot be a situation in which the core positions of the multi-core fibers are completely the same.
For example, in a case of constructing a new transmission path, it is possible to consistently use the same fibers. However, in a case of repairing or extending an existing transmission path, there is a high probability that fibers having different biases of deviations of the core positions will be mixed, such as fibers from different manufacturers or fibers manufactured by different methods. Even if the core positions in the specifications of two optical fibers are the same within an allowable tolerance range, when optical fibers are actually made to face to each other, since the tendencies of the deviations of the cores in both the optical fibers are opposite to each other, there is a possibility that the axial deviation becomes large. For example, when a distance between the cores in a certain optical fiber is shorter than that in the specifications, in a case of splicing the optical fiber to an optical fiber having the same short distance between the cores, the axial deviation does not easily increase. However, on the contrary, in a case of splicing the optical fiber to an optical fiber having a long distance between the cores, there is a possibility that the axial deviation may exceed the allowable tolerance. In addition, even when multi-core fibers are manufactured by the same manufacturing method by the same manufacturer, in a case where the manufacturing lots are different from each other or the longitudinal positions are different from each other, deviations of the core positions may be different, and thus, there is a possibility that the minimum average loss will not be able to be obtained.
As described above, there have been many problems in the related art from the viewpoint of versatility.